Chemical assays, particularly biochemical assays, must often be performed on a large number of samples taken from test tubes or other sample containers. Further, it is often necessary to perform different assays on each of the many samples, each assay requiring mixing of one or more reagents with a sample and performing an analysis such as an optical analysis. Performing such multiple assays on multiple samples manually is very time consuming and subject to error. More recently, robotic systems have been developed to automatically withdraw sample liquid from sample containers and reagent from reagent containers and to mix the two in test cells for analysis. The sample containers and test cells may be positioned in circular arrays on turntables. One or more probes on arms carry sample liquid to the test cells and reagent from reagent containers to the test cell.